In asynchronous digital two-way radio systems, the radios are not synchronized to each other or to any infrastructure, in time or frequency. In typical operation, a target radio obtains both time and frequency synchronization information from the originator radio at the beginning of each call. The radios then communicate using frequency hopping in license-free bands, such as the Industrial, Scientific and Medical (ISM) radio bands. In such systems, the radios connect in a direct talk-around mode as long as the originator and the target are within range of each other. No other infrastructure is required to support the operation of the radios in this mode as long as the radios remain within range of each other.
The range of communication in such radio systems can be increased by introducing a repeater. The repeater unit acts only in an on-demand fashion and does not affect the default asynchronous operation of the radio. An originator radio connects to the target radio via the repeater only when the direct talk-around mode is not possible due to range limitations. In such systems, to relay a call that cannot otherwise be completed in the direct talk-around mode, the repeater also has to acquire time and frequency synchronization information from the originator radio. Following receipt of the information, the repeater has to send new time and frequency information to the target.
A typical repeater based communication scenario is shown in FIG. 1. Communication system 100 is shown along a timeline 102 and includes an originator radio 104 which first attempts to connect to a target radio 110 in direct, talk-around mode by sending initial time and frequency synchronization information 106. If the originator 104 receives no response from the target 110, the originator makes a second attempt, but this time, trying to connect to a repeater 116. In addition to sending synchronization information, the originator 104 also sends identification information to the repeater identifying the target with which the originator wants to connect.
Once a link (referred to as an uplink) is established between the originator 104 and the repeater 116, the repeater sends new time and frequency synchronization information 118 to the target 110. Hence, synchronization information is sent twice from the originator 104, once to the target 110 and then to the repeater 116. This procedure significantly extends the call connect time, which is critical in voice communication in two-way radio systems.
Accordingly, it would be desirable to reduce the call connect time in a digital two-way communication system.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.